Japanese speakers are known to perceive "illusory vowels" within consonant clusters illicit in their language. The present study examines how this perceptual vowel epenthesis is affected by first language (L1) processes (restoration of vowels devoiced through Japanese high vowel devoicing), L1 representations (loanword representations in Japanese speakers’ lexicons), and proficiency in English.

The participants judged the presence or absence of a mora (e.g., ム /mu/) in an auditorily presented English word (e.g., homesick). The 40 test items contained a heterosyllabic consonant cluster with four different voicing patterns to examine whether the vowel restoration process is related to vowel epenthesis. Twenty of the test items are frequently used as loanwords in Japanese, meaning that they are stored in the L1 lexicon with a vowel inserted inside the consonant cluster (e.g., /hoomusikku/). The other 20 are low-frequency items that are virtually nonwords for the non-native participants.

The vowel epenthesis rates and reaction times (RTs) were obtained from 14 introductory learners, 15 intermediate learners, and 19 native speakers.

The results show the main effects of Voice, Loanword Representation, and Proficiency, as well as the interaction among the three factors. Negative correlations between vowel epenthesis rates and RTs were also observed for the learners. The results indicate differential effects of vowel restoration and loanwords on perceptual epenthesis by learners of different proficiency levels.

The present study was one of the first attempts to test the relation between proficiency and perceptual vowel epenthesis using real English words.

The findings demonstrate the robustness of L1 processes and representations in second language perception while substantiating the existing argument for early vowel epenthesis. They also raise questions regarding the effects of training and the role of native speaker input.